Well logging provides a detailed record (through a well log) of the geologic formations (e.g., carbonate rock) penetrated by a borehole. It has been extensively used as a mapping technique for exploring and characterizing the subsurface and evaluating the hydrocarbon production potential of a reservoir, along with the identification of other properties of the formation. Well logging provides useful measurements that may be used to extract information about the rock formation related to, for example, porosity, lithology, potential presence of hydrocarbons, and pore-filling fluids. Measurement techniques are based on at least three broad physical aspects: electrical, acoustic (which includes sonic), and nuclear.
The first logging technique measured the electrical conductivity of a formation and used electrodes. The original induction electrical logging tool had a transmitter (magnetic dipole) and a receiver. The magnetic field from the transmitting dipole induced ground loop currents in the surrounding formation that gave rise to an alternating magnetic field that was sensitive to the formation conductivity. The induced alternating magnetic field was detected by the receiver and the conductivity of the formation through which the signal had passed could be determined. For instance, a reservoir formation filled with hydrocarbon could be recognized on a typical electrical log since it was more resistive than the salt water that was commonly found in deeply buried reservoir rocks. The first well log dates back to 1927, performed in the Pechelbronn field in Alsace, France. Since that time, research and engineering efforts have improved this technology to accommodate harsh well conditions and to investigate complex reservoir properties.
An acoustic or sonic logging tool transmits a sound pulse into the formation that is subsequently detected by a receiver. The speed at which the sound (i.e., acoustic wave) propagates through the formation depends at least in part on the formation's mineral composition and porosity. The measured travel time allows one to determine a sonic velocity that can be used to determine the porosity via the well-known Wyllie time-average relation.
Another logging tool designed for formation evaluation uses Gamma rays and neutrons to characterize the geological formation. The absorption of Gamma radiation is proportional to the density of the formation, while that of neutrons is proportional to the amount of hydrogen present. Gamma ray and neutron logs can be indicative of the porosity distribution.
Production logging tools include a variety of sensors that are used to identify the nature and behavior of fluids in or around the borehole during production. They provide useful information such as temperature, flow rates, and fluid capacitance/impedance. Surveys may be performed during production operations to evaluate the dynamic well performance (i.e., the productivity of different zones) and to diagnose possible well problems.
Logging while drilling (LWD) tools allow for detailed formation evaluation as the well is drilled. This allows one to maximize the reservoir value. LWD tool logs allow drilling engineers to make appropriate decisions for particular realized drilling circumstances and optimally direct the direction of the drill. Different measurements are available using LWD technology and their selection depends on the complexity associated with the mineralogy, texture, and open fractures within a target zone near the wellbore. The measurements tools may include Gamma ray tools, electrical resistivity propagation tools, acoustic/sonic logging tools, neutron porosity tools, and nuclear magnetic resonance (NMR) tools.
Acidization is used extensively in well stimulation operations to increase the permeability of carbonate rocks, thus facilitating the flow of oil to the wellbore. As acid is injected into the porous medium (carbonate rock), highly-permeable channels or “wormholes” are formed by the dissolution of carbonate material. A successful matrix treatment produces thin, but deep wormholes with a minimal amount of injected acid.